1. Field of the Invention
The present invention relates, generally, to the field of defibrillators. More particularly, it relates to an improvement in the efficacy of cardioverter-defibrillators (ICD). Automatic implantable cardioverter-defibrillators (AICDs) or ICDs customarily include a sensor and sensing circuit to determine when a therapeutic shock is needed, a control circuit to determine what type of therapeutic shock is appropriate, a long-term energy source, such as a battery, a short-term energy storage means such as high-voltage capacitor, and a circuit for transferring electrical energy first from the battery to the capacitor and then from the capacitor to the heart by discharging the capacitor in waveforms having particular shapes, durations, and sequences, to electrodes which deliver the energy as a shock to a heart which is to be converted.
Development of implantable cardioverter-defibrillators since their introduction in the mid 1980s, has not only been directed toward improving their reliability in terms of delivering defibrillation pulses when fibrillation of the heart is detected, but also toward increasing their efficacy. That is, to apply to a heart the minimum amount of energy necessary to ensure conversion. By decreasing the amount of energy required for conversion or defibrillation, the physical size of the implanted automatic defibrillator can be decreased by reducing the physical size of the battery, the capacitor, and other components. A decrease in energy requirements also means that even if the defibrillator is with some degree of frequency called upon to defibrillate a heart, the battery will have a longer life. Thus, extending the period of time before which the defibrillator must be replaced.
Advances in reducing the energy required for defibrillation have been made in the past in various ways. The electrodes delivering the defibrillation shocks to the heart have been improved. It has also been found that shocks of particular shapes, durations, and polarities are more effective in defibrillating the heart. This invention relates to further improvements in the shape of the shocks.
2. Description of Related Art Including Information Disclosed Under Secs. 1.97-1.99
In a paper entitled: DECREASED DEFIBRILLATOR-INDUCED DYSFUNCTION WITH BIPHASIC RECTANGULAR WAVEFORMS; by Janice L. Jones and Ronald E. Jones, AM. J. Physiol. 247 (Heart Circ. Physiol. 16); H792-H796, 1984, a study is reported on the characteristics of the negative second portion of a biphasic waveform which best ameliorates postshock dysfunction. The study was based on the use of chick embryo cultured myocardial cells. The article concluded that "the negative tail can only partially reverse the deleterious effects of the leading portion of the waveform and that this effect can be produced either by a low amplitude undershoot that lasts for a long time or by a higher amplitude undershoot that lasts for a shorter time." While this work did find that some waveshapes with the second phase longer than the first reduced dysfunction, dysfunction has never been shown to have a impact on defibrillation efficacy.
The paper Improved Defibrillation Thresholds with Large Contoured Epicardial Electrodes and Biphasic Waveforms; by Ellen G. Dixon, circulation 76, No 5 1176-1184, 1987, is primarily concerned with the testing of large contoured patch electrodes on dogs. The electrodes were tested with monophasic, biphasic, and triphasic waveforms. Further, the biphasic waveforms were tested with the first phase being both longer than and shorter than the second phase. It is reported that biphasic waveforms, with the durations of the first and second phases equal, have a significantly lower threshold voltage than a monophasic waveform.
Furthermore, the defibrillation threshold voltage and energy were reported to be significantly higher for biphasic waveforms in which the relationship of the duration of the first to the second phase were 25/75 and 35/65, compared to 50/50, even though the initial voltage of the first phase was of a greater magnitude than the second, with the trailing voltage of the first phase being equal to the beginning voltage of the second phase.
Thus, while this paper was primarily directed to research with respect to electrodes, it does present data indicating that the duration of the first phase of a biphasic waveform should be equal to or longer than that of the second phase. This conclusion was based upon an earlier postulation by Jones et al that the first phase conditions the heart cells to allow more effective defibrillation by the second phase.
The paper, Comparative Efficacy of Monophasic and Biphasic Truncated Exponential Shocks for Nonthoracotomy Internal Defibrillation in Dogs; by Peter D. Chapman, et al, Journal of the American College of Cardiology, Vol. 12, No. 3, September 1988 pages 739-745, reports the efficacies of monophasic and biphasic truncated exponential shocks in dogs. The monophasic shocks were compared with biphasic shocks having relative P.sub.1 (first phase) versus P.sub.2 (second phase) durations of (50 and 50%, 75 and 25%, 90 and 10%, 25 and 75%, 10 and 90%) It was concluded that biphasic shocks with P.sub.1 (initial positive phase) longer than P.sub.2 (terminal negative phase) markedly reduced energy requirements for nonthoracotomy canine defibrillation and may, therefore, facilitate development of nonthoracotomy devices for clinical applications. The paper further reports that biphasic pulses with the second phase longer than the first phase (25 and 75% and 10 and 90% configurations) resulted in energy thresholds that were significantly higher than even those for monophasic shocks.
The paper: Optimal Biphasic Waveform Morphology for Canine Cardiac Defibrillation with a Transvenous Catheter and Subcutaneous Patch System, by John C. Schuder, et al, Circulation, vol 78, II-219, 1988 set forth that previous studies have shown that biphasic waveforms are generally superior to monophasic waveforms for achieving canine ventricular defibrillation. It further reports on additional tests directed at determining the significance of the duration of the initial phase. All of the tests for this study were conducted with a 10 millisecond truncated exponential waveform shock, and with the final current equal to 25% of the initial current. The timing of the polarity reversal was changed such that initial pulse durations of 1, 3, 5, 7 and 9 milliseconds were tested. The study concluded that ten millisecond biphasic truncated exponential waveforms are more effective with an initial pulse duration of 5 to 7 milliseconds, i.e., equal to or greater than the duration of the second phase.
U.S. Pat. No. 4,850,357--issued Jul. 25, 1989, and entitled: BIPHASIC PULSE GENERATOR FOR AN IMPLANTABLE DEFIBRILLATOR; is directed toward a circuit for delivering biphasic pulses without the need to short circuit the high voltage capacitor, which stores the energy for the pulses, at the end of a pulse. While not elaborating on the relative durations of the first and second phases of the pulse, the phases are shown to be equal, with the initial voltage of the second phase being equal to the terminal voltage of the first phase.
U.S. Pat. No. 4,821,723--issued Apr. 18, 1989, and entitled: BIPHASIC WAVEFORMS FOR DEFIBRILLATION is directed toward a method and apparatus for defibrillating a heart with a biphasic shock having an initial phase, the duration of which is at least slightly greater than the duration of the second phase. Further, the first phase of the biphasic waveform commences with a voltage magnitude equal to or greater than the initial voltage level of the second phase.
Referring to the paper: Transthoracic Ventricular Defibrillation in the 100 Kg calf with Symmetrical One-Cycle Bidirectional Rectangular Wave Stimuli; IEEE Trans Biomed. Eng. 30: 415, 1983, and to the paper: Defibrillator of 100 Kg Calves with Asymmetrical, Bidirectional Rectangular Pulses; Cardiovasa Res. 419, 1984, it is stated in U.S. Pat. No. 4,821,723 that: "Schuder and his associates were able to defibrillate 100 Kg calves using symmetrical biphasic rectangular waveforms at a lower range of energy and current, and to achieve a higher percentage of successful first shock defibrillations than with monophasic waveforms. Those same investigators obtained good results with asymmetrical biphasic waveforms in which the amplitude of the second phase of the shock was smaller than that of the first phase, and the two phases were of equal duration." This patent also sets forth the theory that the duration of the first phase of a biphasic waveform may have a significant effect on the extent of conditioning. It is further stated: "It appears that a short first phase, relative to the second phase, may be of insufficient duration to allow a conditioning process to be completed." As was previously set forth with respect to the Dixon paper, this study is based on the earlier postulation by Jones et al that the first phase conditions the heart cells to allow more effective defibrillation by the second phase.
Other efforts to reduce the size of an implantable defibrillator have been directed toward improvement of the electrodes through which pulses are applied to the heart for defibrillation purposes. U.S. Pat. No. 4,953,551--issued on Sep. 4, 1990 and entitled: METHOD OF DEFIBRILLATING A HEART, is primarily directed toward an improvement in the electrodes. However, the patent also advocates the use of an asymmetrical biphasic waveform. The asymmetrical waveform set forth is one in which the first and second phase, are of equal duration, but in which the initial voltage of the second phase is equal to the final voltage of the first phase, (voltage decays during the pulses on an exponential basis).
U.S. Pat. No. 4,998,531--issued on Mar. 12, 1991, and entitled: IMPLANTABLE N-PHASIC DEFIBRILLATOR OUTPUT BRIDGE CIRCUIT discloses a means for generating not only biphasic, but also monophasic, multi-phase or sequential defibrillation pulses. The patent is not particularly concerned with, nor does it discuss, the efficacy of biphasic pulses nor is it concerned with the relative durations of the first and second pulses.
U.S. Pat. No. 5,083,562 issued on Jan. 28, 1992, and entitled: METHOD AND APPARATUS FOR APPLYING ASYMMETRIC BIPHASIC TRUNCATED EXPONENTIAL COUNTERSHOCKS, sets forth a defibrillation therapy in which a first truncated exponential waveform of a first polarity has a first phase start amplitude and a first phase end amplitude and, a second truncated exponential waveform of second polarity opposite that of the first polarity, has a second phase start amplitude and a second phase end amplitude. The second phase start amplitude being lower than the first phase start amplitude and in a disclosed embodiment being substantially equal to the first end amplitude. Further, the second phase start amplitude is equal to substantially one half of the first phase start amplitude. This patent further teaches that the first and second phases are preferably of equal duration.
While there are many patents and papers in addition to those set forth above which relate to biphasic waveforms, the inventor is unaware of any which teach the advantage of the first phase being shorter than the second.